Mouse mammary cells have a limited growth span when serially propagated in animals even under conditions which are optimal for growth, survival, and function; this decline in proliferative potential is interpreted as an expression of senescence in vivo at the cell and tissue level. This aging phenomenon is related primarily to the number of cell divisions undergone rather than to the passage of time. Mammary aging can be modified by altering the hormonal environment of the tissue, but unlimited growth requires transformation into a precancerous state. This project is designed to further develop the mouse mammary transplant system as a model system for aging studies. We are undertaking a series of investigations aimed at identifying those mechanisms that are responsible for, or are significantly involved in, mammary aging. A related goal is to place special emphasis upon experimental approaches that provide meaningful comparisons with in vitro studies. Accordingly, investigations are being carried out which analyze the pattern of cell proliferation during growth and aging, leading to information on the maximum number of population doublings of which mammary cells are capable. Related studies are in progress on cell cycle changes and changes in size of the proliferating cell pool. Cell and tissue interactions both within mammary epithelial tissues and between the epithelium and its surrounding stroma will be studied. The role of mammogenic hormones as regulatory substance involved in mammary aging will be studied, with special emphasis upon estrogen.